It is generally known that pressure-sensitive adhesives (PSAs) can be applied to the back of a face material, known as a face sheet or face stock, and subsequently be laminated to the silicone coating of a backing sheet to form a pressure-sensitive adhesive laminate, referred to herein simply as a laminate. The adhesive can be applied onto a siliconized backing and then sandwiched onto the face material by lamination. These laminates typically have four layers: a face sheet, a PSA, also referred to herein as an adhesive, a silicone release coating and a backing sheet. The cured silicone is typically bonded strongly to the backing sheet to form what is collectively called a release liner, and the PSA bonds strongly to the face sheet. The PSA and silicone layers, however, bond relatively weakly and can be pulled apart easily.
Typically, it is desirable for the release liner to be easily removable, thereby exposing the free adhesive surface. Therefore, the release strength, which is a measure of the amount of force (the "release force") required to remove the release liner, is generally very low. However, sometimes it is desirable to have higher release strength, or even permanent bonding of the release liner to the face sheet, in selected areas.
The patent literature discloses PSA laminates where the release strength is purported to be "controlled" by altering the composition of the adhesive or the release liner. By exposure of the silicone layer to UV light, bonding of the adhesive-silicone interface can be strengthened for some combinations of adhesive and silicone. Once the composition is manufactured and cured, however, the release strength can no longer be altered. Furthermore, there is no incremental control of release strength.
For example, Reding, U.S. Pat. Nos. 4,952,657, Kobayashi, 5,198,476, and Kessel, 5,217,805, each discloses a UV-cured silicone release coating with a release strength that is controllable by modiifing the silicone composition.
Yoshimura, U.S. Pat. No. 5,560,796, discloses UV curing an adhesive after applying it to a substrate. Although it also discloses coating the substrate with a silicone solution, it does not suggest controllable release of adhesive-silicone interface bonding based on relative amounts of UV exposure.
Another problem encountered in the prior art is that control of the geometric configuration of the areas of specific release strength is difficult to attain. Yet another problem encountered in the prior art is that control of a gradient of release in a specific zone of the PSA laminate is difficult to attain.